1. Field of the Invention
The present invention relates to an image composition method or the like for producing an image signal having a wide dynamic range by merging together a high-sensitivity image signal and a low-sensitivity image signal, which have been obtained by means of photographing a subject, and more particularly, to an image composition method, a solid-state imaging device, and a digital camera, which prevent a composite image from becoming unnatural in the vicinity of a saturation point of the high-sensitivity image signal.
2. Description of the Related Art
An image photographed by a solid-state imaging device using a solid-state imaging element typified by a CCD image sensor or a CMOS image sensor has a shortcoming of a narrow dynamic range. For this reason, a photographic scene involving a wide dynamic range; for instance, optical images such as an image of a person or an afternoon garden observed by way of window panes from a dark, indoor location, cannot be acquired well.
Therefore, as described in JP-A-6-141229, the related-art digital camera is equipped with a solid-state imaging element capable of controlling a period of accumulation of electric charges. An image signal obtained as a result of a subject having been exposed for a long period of time (hereinafter called a “long-exposure image signal”) is read, and an image signal obtained by exposing the same subject for a short period of time (hereinafter called a “short-exposure image signal”) is continuously read. These two image signals are subjected to merging operation, to thus produce an image of the subject having a wide dynamic range.
According to the related-art technique described in JP-A-59-210775, there is used a solid-state imaging element, wherein half of an array of pixels is formed as high-sensitivity pixels and a remaining half is formed as low-sensitivity pixels. An image signal obtained from the high-sensitivity pixels and an image signal obtained from the low-sensitivity pixels, both pixels having been obtained during the same exposure time, are merged together, thereby producing an image of the subject having a wide dynamic range.
A long-exposure image signal output from the solid-state imaging element or an image signal obtained from the high-sensitivity pixel is called a high-sensitivity image signal. Moreover, a short-exposure image signal or an image signal obtained from the low-sensitivity pixel is called a low-sensitivity image signal.
In a case where the high-sensitivity image signal and the low-sensitivity image signal are merged, if the image signals output from the solid-state imaging element are merged together, the number of bits required to perform merging operation is increased. As a result, the amount of look-up table data having a gamma characteristic is also increased, which in turn renders a signal processing circuit for performing signal processing larger in scale and makes the digital camera disadvantageous in terms of cost.
For theses reasons, as described in JP-A-2002-135787, the following method is suitable for practice. Namely, according to this method, the high-sensitivity image signal and the low-sensitivity image signal are subjected to gamma characteristic conversion (i.e., nonlinear processing) through use of different gamma characteristics. Subsequently, the thus-processed signals are added together, thereby merging the signals.
FIG. 9 is a view showing an example of this signal, wherein a horizontal axis represents an incident light exposure level, and a vertical axis represents a signal level. The high-sensitivity image signal H rises comparatively sharply from a nominal level of incident light exposure. Hence, a nominal change in the incident light exposure level is also reflected as an image signal. However, the high-sensitivity image signal has a drawback of saturation arising when the incident light exposure level has reached a predetermined value “t”, thereby causing a loss in color of an image of a bright subject.
In contrast, the low-sensitivity image signal L does not cause a loss in color of the image of the subject even when the incident light exposure level is high. The low-sensitivity image signal L has a drawback of the black color of the image of the subject becoming solid if the incident light exposure level is low. A composite signal G into which the two image signals H, L are merged reflects a change in the incident light exposure level as a change in the image signal G, regardless of whether the incident light exposure level is high or low. For this reason, the chance of occurrence of a loss in white color or occurrence of solid black becomes low, thereby enabling acquisition of an image having a wide dynamic range.